1. Field of the Invention
The present invention relates to electrolytes for electrolytic capacitors and electrolytic capacitors using the said electrolytes.
2. Description of the Prior Art
Amine salts of phthalic acid or maleic acid are generally used as solutes contained in electrolytes for low voltage electrolytic capacitors with a rated voltage in the range of 6.3 to 100 V. Examples of the use of boron complexes of oxalic or maleic acid for this purpose are disclosed in Japanese Laid-Open Patent Publications Nos. 1-194313 and 1-157514. Adipic acid or its ammonium salt is used as a solute contained in electrolytes for intermediate voltage electrolytic capacitors with rated voltages in the range of 160 to 250 V. However, these electrolytes have the disadvantages of low specific conductivity as well as a further pronounced decrease of specific conductivity when exposed to high temperatures. With a view to avoiding this defect, examples of the use of quaternary ammonium salts of boric acid for this purpose are disclosed in Japanese Laid-Open Patent Publication No. 63-1021, while examples of the use of quaternary ammonium salts of catechol complexes of boric acid for this purpose are disclosed in U.S. Pat. No. 3,403,304. As solutes for the electrolytes of high voltage electrolytic capacitors with rated voltages in the range of 350 to 500 V, 1,6-decanedicarboxylic acid or its salts are generally used.
However, if amine salts of phthalic or maleic acid, or boron complexes of oxalic acid, maleic acid or catechol are used as the solute, then, since the sparking voltage is low, the electrolyte cannot be used in capacitors with rated voltages in the range of 6.3 to 100 V. Moreover, if quaternary ammonium salts of boric acid are used, then, since the solubility with respect to aprotic solvents is low, various shortcomings arise, such as precipitation under low temperature conditions, etc. Furthermore, if alcoholic solvents such as ethylene glycol, etc., are used to dissolve these solutes, then esterification reactions proceed at high temperatures, resulting in pronounced increases in the water content of the electrolyte. This water in turn reacts with the aluminum composing the electrodes of capacitor elements, which causes marked deterioration of electrical characteristics and reduction in lifetime of the capacitors.
If 1,6-decanedicarboxylic acid or its salts are used as solutes for the electrolytes for use in electrolytic capacitors, then the specific conductivity is low, and various problems arise, in particular, the impedance characteristics of the capacitor display marked adverse effects at low temperatures.